The present invention relates to a demand defrost control system for a refrigeration apparatus.
Defrost controllers for automatically-defrosting refrigerators periodically interrupt operation of the refrigeration system and energize a heater to defrost the refrigerant evaporator. It has been recognized that maximum energy efficiency may be realized if the interval between automatic defrosting operations is varied according to actual need. Control systems which attempt to vary the interval between defrosting operations according to actual need are generally termed "demand defrost" systems. If successfully implemented, the result is energy savings with no decrease in performance.
One approach to a demand defrost system is to measure the actual amount of frost buildup on the refrigerant evaporator, and to initiate an automatic defrosting operation when the frost buildup becomes excessive. Systems attempting this approach have for example employed mechanical probes, photoelectric sensors, airflow impedance sensors, or sensors responsive to temperature differences between parts of the refrigeration system.
Direct measurement of frost buildup has proved to be difficult, and various predictive type demand defrost systems have been developed as an alternative. Predictive type systems have taken into account such parameters as ambient humidity, refrigerator door openings and total accumulated compressor running time to predict the rate of frost buildup on the evaporator and thus the required time interval between successive automatic defrosting operations.
Any single predictive approach, such as taking into account ambient humidity, may by itself lead to significant inaccuracies. However, by combining several such approaches in a comprehensive system with appropriate weighting of their individual effects, good results may be obtained under most conditions of usage.
The present invention is one approach to a predictive demand defrost system. The invention may be used either by itself, or in combination with other approaches in a comprehensive system. The present invention itself comprehends two separate approaches, one of which may be employed as an override for the other.
In one particular prior art defrost control system, there is a defrost control timer having a cam-operated switch. The arrangement is such that the switch is in a normal position for approximately six hours of timing motor running time, and in a defrost position for approximately twenty minutes of timing motor running time. When the cam-operated switch is in a normal position, energization of the refrigeration system compressor occurs whenever called for by the refrigerator thermostat. In the defrost position, the refrigeration compressor is de-energized and a heater for defrosting the evaporator is energized. This particular prior art system additionally includes a thermal sensor which is responsive to a predetermined evaporator temperature, for example 50.degree. F., being reached during a defrosting operation. When the predetermined temperature is reached, the heater is de-energized even though the cam-operated switch remains in the defrost position. In most cases, the predetermined temperature is reached before the end of the twenty-minute defrost duration period, and there is a period of time, known as defrost "dwell time," during which neither the refrigeration compressor nor the defrost heater is energized. Such a time delay between de-energi ation of the heater and restarting of the compressor lessens the starting load on the compressor by permitting a decrease in evaporator temperature with a consequent decrease in refrigerant pressure.
In this particular prior art defrost control system, the timing motor is connected to operate only when the refrigerator temperature control thermostat is calling for cooling and energizing the refrigerant compressor. Thus the defrost control timer effectively accumulates compressor running time (with the exception of periods during a defrosting operation when the thermostat is calling for cooling but energization of the compressor is prevented by the defrost control timer). This will be recognized as a form of predictive type demand defrost control system, taking into account the parameter of accumulated compressor running time.